1. Field of the Invention
The invention is related to the field of welding dissimilar metals and in particular to welding aided by the modification of the chemical composition and morphology of the surfaces to be joined.
2. Prior Art
The concept of joining dissimilar metals using intermediate compatable layers, acting as a barrier to the diffusion of undesirable elements from one surface to the other was introduced in the late 1960's, H. E. Pattee, R. M. Evans and R. E. Monroe, "The Joining of Dissimilar Metals", Battelle Memorial Institute, Defense Metals Information Center, DMIC Report #S-16, January 1968. This initial concept was later modified and refined for specific applications and given various names, such as eutectic welding and diffusion brazing, W. A. Osczarski and D. S. Duvall, "Advanced Diffusion Welding Process", New Trends in Materials Processing, SAM Materials Science Symposium, Detroit, Mich., Oct. 19-20, 1974 and P. M. Bartle "Diffusion Bonding: A Look at the Future", British Welding Journal, Vol. 54, No. 7, 1975. In these methods, the intermediate layer was in the form of a foil, an electroplated layer, or a vacuum deposited layer as disclosed by Marancik in U.S. Pat. No. 4,011,982.
With these methods, the diffusion effects of the bond were not fully controlled and the formation of liquid during the joining process often resulted in the formation of brittle intermediate phases, degrading the ductility of the joint. Additionally, the formation of liquid led to uncontrolled consumption of the pieces being joined, thereby resulting in uncontrolled dimensional stability.
The effects of intermediate layer thickness and properties on the strength of the formed joints have been assessed, and it was found that in specific cases the bond strength was an inverse function of the intermediate layer thickness above 100 micrometers, A. M. Bartle, "The Implications of Reducing Bonding Temperature", Advances in Welding Processes, Vol. 1, May 1978 and R. Z. Shron, "The Problem of Gauging the Strength of Welded Joints in which there is a Soft Interlayer", Welding Production, Vol. 9, No. 9, 1962.
Disclosed is a method for modifying the surface chemistry of the parts to be joined which eliminates the problems of the prior art discussed above and results in superior welds between dissimilar metals.